This invention relates to oil burner primary controls for interrupted ignition systems utilizing a photoconductive cell to detect the presence and absence of burner flame, and particularly to an improvement therein which ensures safe operation in the event of a particular type of failure of the photoconductive cell.
Disclosed in U.S. Pat. No. 4,242,081, assigned to the assignee of the present invention, is an oil burner primary control for an interrupted ignition system which utilizes a photoconductive cell. As described therein, the cell is part of a gating circuit for a triac. On a call for heat, the resistance of the cell, in the absence of burner flame, is extremely high and gates the triac on. With the triac on, a relay winding is energized by the secondary winding of a voltage step-down transformer. One set of contacts of the relay is effective to energize the fuel supply means, and another set of contacts of the relay establishes a hold-in circuit for the relay winding and connects the safety heater and triac across a portion of the transformer secondary winding. Also occurring when the triac is gated on, is the generation of pulses in a coupling transformer which enables energizing of the igniter.
In the system described in U.S. Pat. No. 4,242,081, under normal operation, combustion occurs before the safety heater has heated sufficiently to open its contacts, which opening would place the system in a lock-out condition. When burner flame appears, the resistance of the photoconductive cell drops very quickly to a value insufficient to effect continued gating of the triac. With the triac off, the safety heater and the igniter are de-energized, and the fuel supply means remains energized. These conditions persist until the thermostat opens to de-energize the relay winding.
There is one particular abnormal condition, however, in which operation of the referenced system is unacceptable. Specifically, it has recently been observed that a prolonged exposure of the cell to a temperature above its temperature rating may cause a drastic change in its resistance characteristics. More specifically, such prolonged exposure may cause the cell resistance to tend to linger at the resistance value it possesed at the high temperature, and to change its resistance value very slowly instead of essentially instantaneously.
With such a defective cell, the burner-on cycle during which the cell becomes defective will terminate normally when the thermostat opens. If the cell resistance remains at a very low value after the flame goes out and is still low on the next call for heat, the voltage across the cell will not be sufficient to effect gating of the triac. If the cell resistance increases sufficiently, the voltage across the cell becomes sufficient to effect gating of the triac. With the triac on, the relay winding is energized. Energizing of the relay winding causes a drop in the voltage at the transformer secondary winding which causes a corresponding drop in voltage across the cell. If the resistance of the cell is increasing very slowly, this voltage drop will result in the voltage across the cell being again insufficient to effect gating of the triac so that the triac is no longer on. Until the cell resistance increases to the value sufficient to again effect gating of the triac, fuel is flowing and the igniter and safety heater are de-energized. This condition is unacceptable since it can result in an accumulation of large amounts of unburned fuel in the combustion chamber.